How Aging Might Have Evolved

September 26, 2011

A new evolutionary computer model demonstrates how aging might have evolved as an evolutionary adaptation. When the environment is changing rapidly, natural selection can favor a higher population turnover rate, and, under some circumstances, the benefit may outweigh the cost to the individual – who dies of old age and cuts short her reproduction.

In a paper published this week in PLoS One, André Martins[1] of University of Sao Paolo, Brazil, demonstrates this phenomenon and explains why it might be important. The idea that aging might be an evolutionary adaptation looks very attractive from the standpoint of genetics. The rate of aging is programmed into genes, at least some of which seem to be ancient and highly-conserved. Since the 1990s, geneticists have been puzzled to find genes that control aging in yeast cells that have homologs in worms, in fruit flies and lab mice. It is also intriguing that the body seems to be able to slow the process of aging at will, and aging is slowest under harsh conditions, e.g., starvation, when the body would seem to be hard pressed to muster the resources necessary for enhanced repair and maintenance.

Evolutionists dismiss the idea that aging could be adaptive on theoretical grounds. The individual who dies early forgoes a chance to keep reproducing, and field studies have shown that the fitness cost of aging in the wild is surprisingly large. The only conceivable benefits of aging are so weak and so widely dispersed that kin selection or group selection is expected to be quite small compared to the direct cost.

In the Martins model, agents of two types, mortal and immortal, are arrayed on a grid, and offspring are born near to the parent. The effect of “population viscosity” quickly segregates subpopulations of mortal and immortal agents into separate regions of the grid. Because the environment changes rapidly, older agents become “obsolete” and unable to compete with some of the newborns that, by chance, are mutated so as to be better adapted to changed conditions. This leads to higher population turnover in regions populated by mortals, and fitness increases more rapidly among the mortals. Over time, mortals accumulate a fitness advantage, and they are able to push back the frontier that separates mortal from immortal subpopulations

Martins’s result is important because it opens a crack in the theoretical reasoning that has prevented evolutionists from considering the possibility that senescence might be exactly what it appears to be – an ancient evolutionary adaptation that has been preserved by natural selection over the aeons.